A compressor is typically used to boost the pressure in a working fluid by receiving power from a prime mover, e.g. an electric motor or a turbine and applying a compressive force to the working fluid. The working fluid can be a gas, such as air or carbon dioxide, a refrigerant or the like. In some applications, the working fluid is a wet gas. A wet gas is understood as being a gas containing a fraction of a liquid phase, for example in form of droplets or aerosol.
Contaminants, in particular liquid contaminants in the form of liquid droplets in the intake gas flow can cause mechanical failures of the centrifugal compressor. Liquid droplets may accumulate in a stream of gas by condensation as the gas impacts surfaces within the compressor. The liquid droplets can hit the rotating parts of the compressor, in particular the compressor impeller, collide with each other and form larger droplets. A portion of the larger droplets is likely to continue in the gas flow direction of the compressor, while a remaining portion of those larger droplets sticks to the rotating impeller surface. The larger droplets remaining on the impeller surface will coalesce with new droplets impacting the impeller surface and this will increase the dimension of the droplets. Larger droplets will eventually be entrained by the gas flow and represent a high erosive potential risk. Moreover, the liquid film forming on the blade surface of the impeller can become unstable and lead to formation of droplets of larger size that are potentially very harmful from the view point of erosion.
In order to reduce the amount of liquid phase in a wet gas flow before entering a centrifugal compressor, a scrubber is usually provided. FIG. 1 illustrates schematically a compressor arrangement using a scrubber to process a wet gas. The arrangement is indicated with the reference number 1 as a whole. The compressor arrangement 1 comprises a centrifugal compressor 3 provided with a plurality of compressor stages 5. Each compressor stage 5 comprises a compressor impeller 7. The compressor impellers 7 are supported by a common rotor shaft 9 in a casing 11 of the centrifugal compressor 3. A wet gas flow entering at 13 is firstly processed through a scrubber 15. In the scrubber 15, the liquid phase is separated as a liquid condensate in the bottom of the scrubber 15 and removed therefrom through a liquid or condensate pipe 17. The gaseous phase is delivered from the top of the scrubber 15 through a dry gas pipe 19 towards the inlet of the compressor 3. Compressed gas is delivered from a discharge pipe 21, while the liquid phase is delivered by the liquid or condensate pipe 17 to a pump 23 and through a delivery pipe 25. Depending on the kind of application, liquid and gas phases can be then rejoined and combined in a wet flow discharge pipe 27.
FIG. 2 illustrates a prospective view of a compressor 3 of the prior art, with a portion of the casing removed, showing the inner components of the compressor. In the representative prior art centrifugal compressor 3 illustrated in FIG. 2 five compressor stages are provided, each comprising a respective impeller 7. A different number of stages can be employed.
FIG. 3 is a schematic cross-section along the longitudinal axis of the centrifugal compressor 3 according to the prior art of FIG. 2. The cross section illustrates three compressor stages 5. The working medium flow enters the first compressor stage 5 through an inlet channel 19A and flows through the first impeller 7. The compressed gas exiting radially the impeller 7 of the first compressor stage 5 is delivered through a diffuser 31 and a casing bend 33 formed in the compressor casing 11. From there the gas flows further through a return channel 35 and a bend 37 into the subsequent impeller 7 of the downstream compressor stage and so on.
In some embodiments known from the prior art, in order to reduce problems connected to the accumulation and coalescence of liquid droplets in the compressor stages, droplet catchers are used. An example of such droplet catchers is disclosed in WO 2001/0053278. Droplet catchers require particularly complex machining of the impellers. The droplets removed from the main working medium flow must be removed from the compressor casing, and therefore a liquid removal system is required. These systems are complex and expensive. Moreover, removal of the liquid collected in the compressor casing often requires stopping the compressor.
This disclosure pertains to the need to more efficiently processing a wet gas in a centrifugal compressor, in order to remove or at alleviate at least one of the problems connected to the presence of the liquid droplets in the compressor stages.